In general, a solid oxide fuel cell (SOFC) comprises a pair of electrodes (anode and cathode) separated by a ceramic, solid-phase electrolyte. To achieve adequate ionic conductivity in such a ceramic electrolyte, the SOFC operates at an elevated temperature, typically between about 750° C. and 1000° C. The material in typical SOFC electrolytes is a fully dense (i.e. non-porous) yttria-stabilized zirconia (YSZ) which is an excellent conductor of negatively charged oxygen (oxide) ions at high temperatures. Typical SOFC anodes are made from a porous nickel/zirconia cermet while typical cathodes are made from magnesium doped lanthanum manganate (LaMnO3), or a strontium doped lanthanum manganate (also known as lanthanum strontium manganate (LSM)). In operation, hydrogen or carbon monoxide (CO) in a fuel stream passing over the anode reacts with oxide ions conducted through the electrolyte to produce water and/or CO2 and electrons. The electrons pass from the anode to outside the fuel cell via an external circuit, through a load on the circuit, and back to the cathode where oxygen from an air stream receives the electrons and is converted into oxide ions which are injected into the electrolyte. The SOFC reactions that occur include:
Anode reaction:H2+O═→H2O+2e−CO+O═→CO2+2e−CH4+4O═→2H2O+CO2+8e−
Cathode reaction:O2+4e−→2O═method comprises multiple concentric tubular layers, namely an inner electrode layer, a middle electrolyte layer, and an outer electrode layer. The inner and outer electrodes may suitably be the anode and cathode respectively, and in such case, fuel may be supplied to the anode by passing through the tube, and air may be supplied to the cathode by passing over the outer surface of the tube.
As mentioned, solid oxide fuel cells operate at high temperatures. It is known that decreasing the wall thickness or increasing the conductivity of the electrolyte will enable the fuel cell to operate at lower temperatures. Reducing the overall wall thickness of the fuel cell has additional benefits, such as reducing the thermal mass and increasing the thermal shock resistance of the fuel cell, which contribute to reducing fuel cell start up/shut down time. Furthermore, reducing the wall thickness in conjunction with the overall fuel cell diameter reduces the size of the fuel cell and enables it to operate in small-scale power applications, such as in laptops, cell phones and other small portable electronic devices. Small-scale fuel cell systems, popularly known as “micro fuel cell” systems, that are currently being developed typically employ direct methanol fuel cell (DMFC) or polymer electrolyte membrane (PEM) technologies. Solid oxide fuel cells have characteristics that make them excellent candidates for micro fuel cell applications, such as having one of the highest energy conversion efficiencies of all fuel cell technologies, typically in the order of 35-60%. However, reducing the wall thickness of an SOFC reduces its mechanical strength, and increases its fragility. Known tubular SOFC stack designs all employ relatively large fuel cells, typically having diameters greater than 5 mm. Such fuel cells also have at least one relatively thick layer—e.g. the anode layer in an “anode supported” fuel cell—that provides mechanical support and structural integrity to the fuel cell. Such large-diameter thick-walled SOFC tubes are not particularly suitable for small-scale applications.
It is therefore desirable to provide a fuel cell with a reduced wall thickness. It is also desirable to provide a small-diameter, thin-walled fuel cell that is suitable for small-scale power applications.